We have identified a new gene family that is important in the regulation of thymocyte expansion and development, and may belong to a new class of tumor suppressor genes. Using subtractive hybridization we isolated a set of genes that are preferentially upregulated in thymocytes that have undergone positive selection. One of these genes named Schlafen exhibits several characteristics that strongly implicate it in the control of physiological thymocyte expansion and development. Sfn is not expressed in early thymocytes, it is expressed at low levels in CD4+CD8+(DP) thymocytes, and dramatically upregulatedin CD4+ or CD8+ positive mature thymocytes. It is expressed in T lymphocytes, but not in liver, heart, and brain. The induced expression of Sfn in fibroblasts causes a rapid and reversible cessation of growth. Similarly, ectopic expression of Sfn early in the T lineage profoundly alters progression of development.The number of thymocytes is approximately 10-30% of normal, the T cell antigen receptor is expressed at very low levels on the cell surface, and DP thymocytes fail to complete maturation. The conceptual protein sequence from Sfn has no known similarities or consensus domains. Since we have no framework to understand these effects, there must be basic aspects of T cell development and its regulation that we have not anticipated. In this proposal we outline a series of experiments that should illuminate this new control point in thymocyte development. We will determine the sequence identity and relatedness of Sfn isoforms in mice and humans, and determine their effects on growth control in fibroblasts and T cells. These studies will lead to further analyses of structure-function relationships. We will produce Sfn-deficient mice by gene targetting, and determine the resulting thymic phenotype. We predict that there will be a disregulated thymocyte expansion. We will determine the cellular localization of Sfn and its effects on the cell cycle. Upon identifying a specific point in cell cycle arrest, we will begin to determine the molecular target of Sfn. Experiments will focus on the Cyclin holoenzymes that are known to regulate specific checkpoints in the cell cycle.